FYI, unless you're providing reproducible builds to F-Droid signed by your key (which doesn't seem to be the case), that APK is going to be signed with a different key. So it's either uninstallable over top of a Play Store-derived APK, or if someone does install it who doesn't currently have Element installed, they won't be able to install a Play Store-derived APK later – at least not without uninstalling first, and unless they do that with the right adb option, they'll lose any app data they have.
Ideally you could set up reproducible builds and make sure that the version in the default F-Droid repo stays up-to-date, but reproducible builds may not be practical for you right now (I'm not sure). Barring that, as you mentioned in the blog post, setting up your own F-Droid repo with self-signed APKs is a good option.
I haven't yet played with Matrix nearly as much as I would like, but I love the vision. Thanks for your efforts!
Also nice plug for F-Droid; they're doing good work as well.
